1. Field of the Invention
The present invention relates to a cutting tool, such as a small drill, which is primarily used for forming deep, small-diameter holes in printed circuit boards.
The present application is based on Japanese patent application No. 2000-346953, the contents of which are incorporated by reference into this specification.
2. Background Art
Generally, small drills are expected to form holes having extremely small diameters. Typical small drills have a rod-shaped cutting portion having a small diameter of about 0.05 to 3.175 mm and a shank portion having a relatively large diameter which is connected to a rotating shaft of a machine tool. The cutting portion and the shank portion are integrally formed, or are connected to each other by soldering, by interference fit, etc. The cutting portion is usually constructed of a cemented carbide, and the shank portion is usually constructed of a cemented carbide, steel, etc.
In conventional small drills, two chip-discharging flutes are helically formed in the exterior surface of the cutting portion from the tip end toward the other end, in a rotationally symmetrical manner around the rotational axis. In such conventional small drills having two chip-discharging flutes, the core diameter thereof is reduced by having the two chip-discharging flutes, and the rigidity thereof is also reduced. Accordingly, when a deep, small-diameter hole, such as a hole having a diameter of 1 mm or less and a ratio of depth to diameter of 5 or more, is formed, the straightness of the formed hole in the depth direction is degraded. Thus, the accuracy of the hole position is reduced, and breakage of the cutting portion may occur. Although such problems may be solved by employing step feeding, the hole-forming speed will be extremely low, and productivity will be greatly degraded.
A small drill which is free from the above-described disadvantages is disclosed in U.S. Pat. No. 5,584,617. FIG. 9 is a side view of this small drill 10, and FIG. 10 is a sectional view of a cutting portion of the small drill 10. The small drill 10 includes a cutting portion 1 and a shank portion. As shown in FIG. 9, a chip-discharging flute 2 is helically formed in the cutting portion 1 around the rotational axis O. In addition, the helix angle “γ” of the chip-discharging flute 2 is continuously increased from the tip end of the cutting portion 1 toward the other end, so that chip dischargeability is improved. In addition, with reference to FIG. 10, a land 3, which is the peripheral surface of the cutting portion 1, includes a margin 4 and a body clearance 5 having a constant depth “a”. The margin 4 is formed in the region directly behind the chip-discharging flute 2 in the rotating direction T of the small drill 10, and the body clearance 5 is formed in a region directly behind the margin 4. Since only one chip-discharging flute 2 is provided in the small drill 10, the core diameter is relatively large. Thus, the rigidity of the small drill 10 is ensured, and the above-described problems are solved to some extent.
In order to ensure the rigidity, however, only one chip-discharging flute 2 is formed in the small drill 10. Thus, space for discharging chips is reduced compared to the conventional small drill which has two chip-discharging flutes. Accordingly, chip dischargeability is degraded, so that the chips may clump together inside the flute and failure to discharge the chips may occur.
In addition, the depth “a” of the body clearance 5, which is formed behind the margin 4 in the rotating direction T, is relatively small. Accordingly, the space inside the body clearance 5 is not sufficient for discharging the chips, so the body clearance 5 does not serve to discharge the chips.
As a result, when a hole having an extremely small diameter, such as 0.5 mm or less, and an extremely large ratio of diameter to depth, such as 10 or more, is formed, failure to discharge the chips may frequently occur. Accordingly, the small drill 10 cannot satisfy an increasing demand to form smaller and deeper holes.